Pros and Cons of low-profile tires? 3

[Retracnic]

A colleague and I were debating large diameter rim/low profile tire combinations. I made the statement that the only reason for large diameter rims was to provide sufficient clearances for larger brake components. And that low profile tires were a necessary evil to accomplish this.

He completely disagreed. His belief, is the use of larger rims is for the sole purpose of running low-profile tires. Then he went on to claim alleged performance benefits.

To me it seems that a lower profile tire would provide less straight line traction, and not be as capable of generating high slip angles. The one benefit I can see is that low-profile tires would be less prone to rolling over in a corner, but I’m not sure.

I would appreciate any input in this debate. I leave the ruling in the court of public opinion.

Thanks,

Bryan Carter

 

[NormPeterson]

One thing that I didn't see was any ref to the tire's load capability.

What is needed (and what I'd also like to see) would be the tables that provide the rated load (lbs) vs the load index and inflation pressure. I've got some ancient tables of similar info, but they date back to when bias-ply tires (unbelted) were still common.

Norm

 

[NormPeterson]

One thing that I didn't see was any ref to the tire's load capability.What is needed (and what I'd also like to see) would be the tables that provide the rated load (lbs) vs the load index and inflation pressure. I've got some ancient tables of similar info, but they date back to when bias-ply tires (unbelted) were still common.

Norm

 

[NormPeterson]

Evidently this site supports neither nor editing of posts already submitted. Sorry for the double post above and this off-topic explanation.

Norm

 

[GregLocock]

I've got current T&RA(Australia) tables, ie in metric. What specifically do you want to know?


Cheers

Greg Locock

 

[NormPeterson]

While it's not too hard to find the maximum load vs load index, i.e.

LOAD LOAD
INDEX (lbs*)
85 1135
86 1168
87 1201
88 1235
89 1279
90 1323
91 1356
92 1389
93 1433
94 1477
95 1521

. . . that table does not provide any data with respect to inflation pressure. The oldie table that I mentioned above looked something like . . .

Tire..... Inflation pressures (psi)
Size..... ..20.. ..22. ..24. ..26.. ..28. ..30. ..32.. . . .
8.25-15 1250 1310 1380 1440 1500 1560 1620 . . .

(Note that the periods before and after the inflation pressure figures are to hopefully make the table line up in columns)

The general reason I'd like to have that information is because it's useful to have handy, particularly if you're going to deviate from OE tire fitment or get involved in such discussions.

The more specific personal reason revolves around my upcoming need to replace the tires on my 2001 Maxima. Tire size is an issue that's been brought up numerous times by others on the Maxima forum to which I contribute, but so much of the *information* on this topic over there is juvenile, anecdotal, and appearance-oriented that it's all but useless to me.

The short version of the story is that the OE tire is the Bridgestone RE92 in 225/50-17 (I'm ignoring the speed ratings), and the OE wheels are 17 x 7, which I understand to be the minimum width for that tire. Not only that, but the size availability table at

http://www.tirerack.com

indicate that there's a dedicated part number for Nissan in that size (I'm guessing that it's for whatever it took to get a 94 load index in that size).

The RE92's aren't very good in the wet as the small sipes start to get shallow even though there's adequate tread depth per the usual measuring methods. So I'm looking for an alternative. There's little to choose from in that size, and most of the tires available in that size are relatively expensive. There's plenty of choice in 235/45-17, though that's at least a half inch outside the rim width guidelines. And there's a wide variety to choose from in 225/45-17, but that size isn't normally associated with LI 94. So I'd be giving up some load capacity at any given pressure, though I should be able to recover that by running a bit more inflation pressure. Yes, I'm willing to trade away ride softness to get there. No, I don't currently have the means to justify scrapping the OE wheels and go aftermarket (besides, I kind of like the OE wheels anyway).

Norm

 

[GregLocock]

Incidentally, according to Autralian T&RA the 225/50 17 can be fitted to rims 6J through to 8J. 7J is the measuring rim. 670 kg at 250 kPa. (101 kPa=14.7 psi)

At 200 kPA ie 30 psi (guessing what the recommended tyre pressure is) the load capacity is 560 kg

225 45/17 will go on a 7J rim, but is only a 90 load rating, 600 kg at 250 kpa.

For 560 kg capacity you'll need an inflation pressure of 230 kPa on the smaller tyre.




Cheers

Greg Locock

 

[NormPeterson]

Thanks, Greg. A 4 psi increase is about what I had been thinking.

FWIW, the Nissan-recommended pressures are 32F/32R, though I've adjusted them slightly (to 36-ish/29-ish) in the interest of more nimble turn-in (and with the knowledge that a full 5-passenger + trunk load is a rare occurrence).

Any idea where one might get a copy of those tables?

Norm

 

[GregLocock]

I've just posted the contact details for T&RA in the FAQ section. I think the book costs about thirty bucks.

Cheers

Greg Locock

 

[tireman9]

See the formula in the "tire size & inflation thread"

35 yrs Tire Eng. Designed basic rain Firestone for CART. SCCA & IMSA Pro & Am. Set lap records at 6 different road courses in '89-91.

 

[pvision]

Regarding tyre sizes;

I have built an online tyre size calculator which has the useful bonus of showing two calculation to compare two different tyre sizes and/or ARs and/or wheel diameters

I'd like to add a third calculator to show similar data (rolling circumference, diameter, etc) for race tyres. Where can I find a tyre size decoder for race tyre sizes such as 240/515-13?

This is a metric description - the imperial equivalent would be 9.5/20.3-13. I'd like to be able to decode both

The tyre calculator is at

http://www.bolide.co.uk/actions/info.taf?page=tyresizecalculator

Nick Froome

 

[carnage1]

there is one very important aspect that everyone has seemed to forget about large rims and low profile tires
anyone remember laws about ground clearance?
no part of the car can be lower than the bottom edge of the wheel at ride height, so if you want a decent size and still be legaly crusing low (important especially to large oem and aftermarket companies) you have to have low profile tires.
So i think the consumers may be emulating the custom car world, you may not have the skill to do a good lowering job but you can buy fancy wheels.

 

[SusTestEng]

One problem I have found with working with low profile and stiff sidewall tires is matching the compliance of the tire and wheel as an assembly. Because of the rather quick grip decay at the limit, the compliance of the wheel is critical to help this situation. This application was an AWD car with an extremly high limit capability. We chose a forged alloy wheel to allow the wheel to bend slightly with the tire's sidewall in order to achieve a more linear grip decay at the limit. This made the driving near limit more predictable, but almost doubled the cost of the wheel.

 

[chapper27]

so what is your conclusion. I think; lower profile, larger wheel, therefore more unsprung weight, therefore straight line performance is decreased with low profile tires.
But what about the true effect in a curve? does the stiffer sidewall compensate for the greater wt?

 

[jadcock]

One thing I've noticed, and heard in amateur circles, is a larger diameter wheel with a lower profile tire usually equates to poorer acceleration and "spongier" braking, due to more rotating mass. I suppose the heavier the wheel, the more energy it takes to get it moving (and to stop it). That large diameter rim displaces the air that you would have had in a higher sidewall tire (if you keep the overall diameter the same), so it's in effect a heavier wheel/tire combo. Perhaps that increased unsprung weight could wreak havoc in other areas, like being harder on the shocks, etc. No?

Jason